Nonlinear spectral dynamics of hypersonic laminar boundary layer flow

The nonlinear interactions of the instability modes in a hypersonic laminar boundary layer undergoing natural transition are examined using bispectral analysis. The data are from an experiment of a boundary layer flow on a co oled-wall cone in a low-level free-stream disturbance hypersonic wind tunne l, and thus, the bispectral measurements are a good representation of the n atural transition processes. The bispectral analysis shows that in the init ial stages of transition the dominant nonlinear interaction is forcing by t he fundamental to generate a harmonic. Subsequently, mutual forcing of the fundamental and harmonic yield a second harmonic. Difference interactions w ithin the band of unstable disturbances centered on the fundamental and har monic also generate a low frequency nonlinear interaction. At high amplitud es of the fundamental and harmonic a nonlinear interaction characterized by a low frequency modulation of the fundamental and harmonic then follows. T his nonlinear interaction is then the most dominant and precedes the breakd own of the laminar flow. (C) 1999 American Institute of Physics. [S1070-663 1(99)02312-0].